Oh, Jaeseok ¹ ; Joharchi, Omid ² ; Lee, Seunghwan ³ and Jung, Chuleui ⁴

¹Laboratory of Insect Biosystematics, Department of Agricultural Biotechnology, Seoul National University, Seoul, Republic of Korea & Research Institute of Agriculture and Life Sciences, Seoul National University, Seoul, Republic of Korea.
²Agricultural Research Institute, Gyeongkuk National University, Andong, Republic of Korea & Anatis Bioprotection Inc., Saint-Jacques-de-Mineur, Québec, J0J 1Z0, Canada.
³Laboratory of Insect Biosystematics, Department of Agricultural Biotechnology, Seoul National University, Seoul, Republic of Korea & Research Institute of Agriculture and Life Sciences, Seoul National University, Seoul, Republic of Korea.
⁴Agricultural Research Institute, Gyeongkuk National University, Andong, Republic of Korea & Department of Plant medicals, Gyeongkuk National University, Andong, Republic of Korea.

2025 - Volume: 65 Issue: 2 pages: 280-305

ZooBank LSID: 9DF1E2C7-A93E-4420-8FD2-5BDF457C2EB9

Original research

Keywords

Abstract

Introduction

The suborder Trigynaspida are group of large-bodied mites that comprise approximately 345 species and 117 genera in 27 families (Kim 2004; Beaulieu et al. 2011; Seeman and Nahrung 2018). Their members are found world-wide, including on subantarctic islands (Hunter 1970) and cool temperate regions (Bank 1904; Franks et al. 1991), but are mostly known from tropical and subtropical regions (Trägårdh 1950; Elsen 1981; Zhang et al. 2017). Generally, most trigynaspid mites forming relationship with social and subsocial insects (e.g. ants, cockroaches, scolytine beetles, passalid beetles) and immature stages live as predators within the host's nest (Seeman 2012; Kinn 1968; Hunter 1993; Seeman 2017; Seeman and Walter 2023). Among the hosts, passalid beetles are home to an astonishing diversity of various mesostigmatan families, such as Diarthrophallidae, Fedrizziidae, Klinckowstroemiidae, and Megisthanidae, which are found only on this family of beetles (Rosario and Hunter 1988; Seeman 2007; Seeman 2017, 2019; Haitlinger 2024). Likewise, the family Hoplomegistidae are also found on passalids only, with six of their ten described species found on unidentified passalid species in Africa, Central America, South America, China. The descriptions of the remaining four species have no mentioned their hosts (Table 1).

Both Hoplomegistidae and Megisthanidae comprise the Megisthanoidea (Kim 2004). The immature stages of Hoplomegistidae are unknown, but the immature stages of Megisthanus floridanus feed on small arthropods, dead or alive, and they continue to feed on this prey are becoming an adult, even though they are closely associated with passalid host (Butler and Hunter 1968). Additionally, the adults Megisthanus thorelli and Megisthanus leviathanicus also behaved in a similar manner, readily accepting squashed Drosophila as prey (personal observation, OD Seeman).

Descriptions of Hoplomegistidae are rudimentary to varying degrees, with the first complete description being that of Hoplomegistus truittae by Butler and Hunter (1968). After 60 years elapsed, two more species were added from China, representing the first record of this family from the Asian continent (Zhang et al. 2017). Since the Hoplomegistidae are poorly studied, and their taxonomy is at least confusing in a nomenclatural sense, we consider it also pertinent to provide a detailed history of the family (Table 2). In this study, we report a new Stenosternum species, representing the first trigynaspid mite in Korea, and the descriptions of larva and nymphal stages of the genus are provided for the first time.

Material and methods

The host beetles (Leptaulax koreanus Nomura, Kon, Johki, & Lee, 1993) were collected from under the bark of rotten logs and frass beneath a stump. Mites were removed with a fine brush into vials of 99.9% ethanol. To obtain the immature stages, some beetles and mites were transferred to a plastic cage (23 X 14 X 18 cm in size) with pieces of tree bark, and a sufficient amount of fermented sawdust (Kumdomi insect ltd., Korea) for rearing insects, and kept at room temperature at least two to three months. Mite specimens were cleared in lactic acid solution and mounted in a PVA medium (Downs 1943). The specimens were examined with an Olympus BX53 compound microscope equipped with DIC (differential interference contrast) optical systems. Photomicrographs and morphological measurements were conducted with an attached camera Olympus DP27 and cellSens Standard software (version 3.1). Selected photographs were stacked using Zerene stacker v.1.04 (Zerene Systems, USA). Line drawing images were prepared using Adobe Illustrator CC 22.0.1 software based on the original photomicrographs. Measurements are expressed as ranges (minimum-maximum) in micrometers (μm). For the dorsal shield, the length was measured from the anterior to the posterior margins along the midline, and the width at the level of s6, respectively. For the sternal shield, the length and width were measured at the maximum length (anterolateral to posterolateral corners) and broadest point (at the level of endopodal region between coxae II and III). The length of the latigynal shields was measured along with the midline from the sclerotized anterior margin of latigynal shields to the posterior margin of the shield, and the width was measured between the anterolateral corners. The length of the legs was measured from the base of coxa to the apex of tarsus (excluding the pre-tarsus). The nomenclature for the dorsal chaetotaxy follows Lindquist and Evans (1965), and setae notations for leg and palp follow Evans (1963) and Seeman (2017). The other morphological structures largely follow Evans and Till (1979), Seeman (2000), and Seeman and Walter (1997). The examined specimens are deposited at the National Institute of Biological Resources, Korea (NIBR). Paratypes are also deposited in the Laboratory of Insect Biosystematics, Seoul National University (SNU), Korea. For the discussion part, with the help of Jing-Lan Sun (Guizhou University), we used microphotographs of paratype S. bifurcata Zhang et al., 2017 specimens deposited at Guizhou University.

Results

Taxonomy

Suborder Trigynaspida Camin & Gorirossi, 1955

Family Hoplomegistidae Camin & Gorirossi, 1955

Genus Stenosternum Kramer, 1898

Hoplomegistus Berlese, 1903.

Syngynaspis Trägårdh, 1937.

Stenosternum, lapsus of Stenosternum, Lindquist et al. 2009.

Type species. Stenosternum bipilosum Kramer, 1898.

Stenosternum leptaulax n. sp.

ZOOBANK: FD68B913-7248-4888-961E-9CDC023E2729

(Figures 1–14)

Material examined

Holotype — Female. Jikdong-ri, Sohol-eup, Pocheon-si, Gyenggi-do, Korea, 25 May. 2019, J. Oh coll., on Leptaulax koreanus Nomura, Kon, Johki & Lee, 1993 (Coleoptera: Passalidae).

Paratype — 1 female, 1 male, Jikdong-ri, Sohol-eup, Pocheon-si, Gyeonggi-do, Korea, 25 May. 2019, J. Oh coll., on L. koreanus; 2 females, 1 male, Minrak-dong, Euijeongbu-si, Gyeonggi-do, Korea, 30 Nov. 2020, J.Y. Sim Coll., on L. koreanus; 1 female, Jikdong-ri, Sohol-eup, Pocheon-si, Gyeonggi-do, Korea, 22 Oct. 2020, J. Oh coll., on L. koreanus; 2 females, 1 male, 7 larvae, 4 protonymphs, 14 deutonymphs, Laboratory rearing condition, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul, Korea, 06 Feb. 2021 – 06 May. 2021, J. Oh coll.

Diagnosis (adults)

Female dorsal shield ovate with 32 pairs of setae and single j2 seta (z4, z6, and J2 setae present); paired presternal shields bear serrated st1 and iv1; sternal shield concaved, trapezoidal-shaped, bearing six pairs of smooth setae; sternogynal shield eyebrow-shaped, with iv2, iv3 located on anterior margin; latigynal shields fused with anterior tip of endopodal plates; ventral shield with seven pairs of setae, distinct pore-like structures on anterior region; metapodal shields each with seven setae, covered with polygonal reticulation; ventrianal shield bearing three pairs of setae, a pair of small shields located postero-laterally. Corniculi distinctively bifurcated. Male with two pairs of short smooth setae on margin of genital opening. Femur II and genu II with spur-like modified setae on ventral portion.

Description (female)

(Figures 1–3)

Five specimens measured.

Dorsal idiosoma — (Figures 1 and 2) — Dorsal idiosoma 1308–1387 long × 898–1038 wide, ovate, reddish brown, anterior margin truncated, round posteriorly; dorsal shield 1098–1122 long × 605–628 wide bearing 32 pairs of setae plus unpaired j2 seta: 17 pairs of setae (j1, j3–j6, z1–z6, s1, s3–s6, and r1) plus single j2 on podonotal region and 15 pairs of setae (J1–J5, Z1–Z5, and S1–S5) on opisthonotal region (Figures 1A and 2A); all setae slightly serrated 51–95, J5 longest 160–202.

Ventral idiosoma — (Figures 1 and 2) — Tritosternum with a pair of pilose laciniae 162–185, columnal base 96–190 long situated on a small, transparent plate. A pair of reticulated presternal shields 67–75 long × 92–113 wide, slightly serrated st1 109–137 and bearing iv1 (Figures 1B, C and 2B). Sternal shield length 147–156 max, 83–87 min, width 316–342 max, 173–198 min trapezoidal-shaped, each margin distinctly concaved, bearing six pairs of smooth sternal setae and iv2: st2 56–68, st3 40–64, st4 22–34, st5 21–31, st6 22–31, and st7 20–31 (Figures 1B, C and 2B). Sternogynal shields paired, each eyebrow-shaped, median margin 105–116 wide, iv3 located on anterior margin of sternogynal shield (Figures 1B, C and 2B). Latigynal shields fused 192–219 long × 281–302 wide, bearing three pairs of smooth setae 49–77, anterior margin notched; anterolateral corners of latigynal shield fused with endopodal plates (Figures 1B, C and 2B). Paired vaginal sclerites well-developed and posteriorly extended with U-shaped medial sclerite (Figures 1B, C and 2B). Opisthogastral plates surrounded by secondary sclerotization to varying degrees (Figures 1B, D and 2B); ventral and metapodal shields covered with polygonal scale-like reticulation; ventral shield 466–515 long × 263–327 wide elongated trapezoidal-shaped, bearing seven pairs of smooth setae 23–107, anterior region with large medial pore-like structure plus several distinct pores; a pair of setae situated on soft cuticle anteromesad metapodal shields (Figures 1B, D and 2B). Metapodal shields 292–335 long × 187–230 wide bearing seven slightly serrated setae 49–74 (Figures 1B, D and 2B). Ventrianal shield 144–154 long × 389–435 wide round trapezoidal-shaped, bearing three pairs of setae; a pair of small shields located posterolaterad ventrianal shield, each shield with a seta and two small pores (Figures 1B, E and 2B). Opisthogastric soft cuticle with about nine pairs of pores and nine pairs of setae, all setae on small platelets (Figures 1B and 2B). Peritremes extended to level of coxa I, peritrematal shield fused with exopodal plates, poststigmatal plates connected with endopodal plates, pore-like structure present posterolaterad stigmatal opening (Figures 1B and 2B).

Gnathosoma — (Figures 1 and 2) — Epistome triangular, margin smooth, ventral median keel well-developed (Figures 1G and 2D). Subcapitulum with four transverse rows of denticles, each row with numerous tiny denticles (Figure 2C); four pairs of hypostomal setae smooth: h3 128–143 > h1 86–101 > pc 61–72 > h2 55–69 (Figures 1F and 2C). Corniculi robust and bifurcated into two parts, horn-like outer part 85–96 and hook-like inner part 26–40 (Figures 1F and 2C). Setal counts of palp digits 323–395: trochanter 2, femur 5, genu 7 (al1 strongly barbed, al2 spatulated spoon-like), tibia 15, most palpal setae slightly serrated except for smooth genu ad, al1, tibia al1, al2, and all tarsal setae; palp apotele two-tined (Figures 1F and 2C). Cheliceral digits well-developed, movable digit 189–193 with three large teeth and a row of small teeth, fixed digit 158–166 with a row of small teeth and three large teeth in apical part; a pair of dendritic excrescences (Figure 1H).

Legs — (Figure 3) — Leg II–IV with well-developed claws and pulvilli, all legs setae slightly serrated. Legs length: Leg IV 1086–1254 > Leg I 1077–1230 > II 768–906 > III 739–887. Chaetotaxy of legs: Leg I (Figure 3A): coxa 2, trochanter 6, femur (12) 2-2/2, 2/2-2, genu (12) 2-3/1, 3/1-2, tibia (14) 3-3/2, 2/2-2. Leg II (Figure 3B): coxa 2, trochanter 5, femur (10) 2-1/2, 2/1-2, genu (12) 2-3/1, 3/1-2, tibia (10) 2-2/1, 2/1-2. Leg III (Figure 3C): coxa 2, trochanter 5, femur (7) 1-2/1, 1/1-1, genu (10) 2-3/1, 2/1-1, tibia (9) 2-2/1, 2/1-1. Leg IV (Figure 3D): coxa 1, trochanter 5, femur (8) 3-1/1, 1/1-1, genu (10) 1-3/0, 3/1-2, tibia (10) 1-2/1, 2/-2; leg IV with macrosetae on genu 186–191 and tarsus 245–253, and small spur on ventral portion 17–24 (Figure 3D).

Description (male)

(Figures 4 and 5)

Two specimens measured.

Dorsal idiosoma — (Figure 4) — Dorsal shield 1258–1317 long × 904–924 wide. Other characteristics same as in female (Figure 4A).

Ventral idiosoma — (Figures 4 and 5) — Large presternal shields 111–119 long × 92–99 wide bearing st1 71–76 and iv1 (Figures 4B, C and 5A). Sternogynal shield 286–291 long × 267–271 wide polygonal-shaped, anterior margin concaved, bearing five pairs of setae and iv2, iv3: st2 29–50, st3 30–49, st4 26–48, st5 30–46, and st6 25–49 and two pairs of additional setae flanking genital opening (one may be asymmetrically off shield); genital opening with two pairs of short smooth setae 6–12, first pair on valves, second pair on margin (Figures 4B, C and 5A). Ventral shield with six to seven pairs of setae 31–80, and six to seven pairs of small pore-like structures on anterior part (Figures 4B, D and 5A). A pair of metapodal shields with five to eight pairs of setae 45–68 (Figures 4D and 5A). Anal shield same as in female (Figures 4E and 5A).

Gnathosoma — (Figure 4) — As in female. Chelicera digit well-developed, movable digit 181–188 with a pair of brush-like excrescences, fixed digit 178–179 (Figure 4H). Shape of epistome same as in female (Figure 4G).

Legs — (Figures 4 and 5) — Legs length: Leg IV 1148–1363 > Leg I 1077–1235 > II 828–928 > III 823–906. Femur II and genu II with swollen, bent, and spur-like modified setae on ventral portion, pv seta on femur II largest 86–94 (Figures 4F and 5B). Four different sized spurs on femur IV 44–50. Leg chaetotaxy same as in female.

Description (larva)

(Figures 6–8)

Six specimens measured.

Dorsal idiosoma — (Figures 6 and 7) — Dorsal idiosoma 419–516 long × 269–357 wide ovate, whitish, and soft cuticle covered with fine ridges (Figures 6A and 7A), bearing 17 pairs of slightly serrated dorsal setae 23–59. Podonotal shield 267–304 long × 204–208 wide weakly sclerotized, bearing nine pairs of setae (j1, j3–j6, z2, z4, z5, and s4); opisthonotal shield absent, opisthonotum bearing eight pairs of setae (s6, J2–J5, Z4, S3, and S4) (Figures 6A and 7A).

Ventral idiosoma — (Figures 6 and 7) — Tritosternum with a pair of pilose laciniae 78–100, columnar base 35–41 long. Ventral idiosoma bearing three pairs of sternal setae: st1 30–35, st2 31–35, and st3 29–35; st3 bearing on small triangular plates and other ventral plates absent. Opisthogaster with four pairs of ventral setae (Jv1, Jv2, Jv5, and Zv2) 19–70; a pair of euanal setae 16–20, a pair of para-anal setae 119–141, and post-anal seta 271–283 present. Peritremes and stigma absent (Figures 6B and 7B).

Gnathosoma — (Figures 6 and 7) — Epistome triangular, margin serrated, ventral median keel present (Figures 6D and 7C). Subcapitulum with six transverse rows of denticles, each row with 19–25 tiny denticles (Figure 6E); two pairs of hypostomal setae smooth: h2 77–86 > h1 36–47 (Figure 6C). Corniculi robust and bifurcated as in adults, horn-like outer part 29–37, hook-like inner part 11–20 (Figure 6C). Setal counts of palp digits 161–240: trochanter 0 (with small ventral projection), femur 4 (ventral seta longest), genu 5 (al thicker), tibia 10, tiny spine on anterior margin of genu, all palpal setae slightly serrated; palp apotele two-tined (Figures 6C and F). Cheliceral digits 92–101 well-developed, movable digit with one large proximal tooth and a row of small teeth; excrescences hair-like (Figure 6G).

Legs — (Figure 8) — Leg II and III with well-developed claws and pulvilli, all legs setae slightly serrated. Legs length: Leg I 491–539 > III 354–419 > II 350–410. Chaetotaxy of Legs: Leg I (Figure 8A): coxa 2, trochanter 4, femur (10) 2-2/1, 2/1-2, genu (8) 1-2/1, 2/1-1, tibia (8) 1-2/1, 2/1-1. Leg II (Figure 8B): coxa 2, trochanter 4, femur (7) 1-2/1, 2/0-1 (pd1 longest), genu (6) 1-2/0, 2/0-1 (pd1 longest), tibia (7) 1-1/1, 2/1-1. Leg III (Figure 8C): coxa 2, trochanter 4, femur (5) 1-2/0, 1/0-1, genu (6) 1-2/0, 2/0-1 (ad1, pd1 longest), tibia (7) 1-1/1, 2/1-1; leg III with macrosetae on genu 96–130 and tarsus 102–128. Ventral intercalary sclerite present on tarsus II and III without setae (Figures 8B and C).

Description (protonymph)

(Figures 9–11)

Four specimens measured.

Dorsal idiosoma — (Figures 9 and 10) — Dorsal idiosoma 595–641 long × 361–413 wide, ovate, whitish, bearing 39 pairs of setae plus unpaired j2 seta. Podonotal shield 306–327 long × 249–274 wide, weakly sclerotized, truncated anteriorly, 10 pairs of smooth setae on shield (j3–j5, z2, z4, z5, and s3–s5) (Figures 9A and 10A). Pygidial shield fused with ventrianal shield 59–84 long × 125–172 wide, posterior tubercles bearing setae J5 128–153 and flanked by plate bearing setae J4, Z4, and S4 (Figures 9A and 10A).

Ventral idiosoma — (Figures 9 and 10) — Tritosternum with a pair of pilose laciniae 84–96, columnar base 42–53 long (Figures 9B and 10B). Sternal shield 228–244 long × 89–98 wide well-developed, bearing a pair of setae st2 30–35; st3 30–37 on soft cuticle, st1 33–42 on small triangular presternal shield; sternal poroids iv1 and iv2 located on soft cuticle flanked sternal shield (Figures 9B and 10B). Opisthogastric cuticle with three pairs of smooth setae (Jv1, Jv2, and Jv5) 36–73 (Figures 9B and 10B). Ventrianal region of fused pygidial-ventrianal shield with three pairs of smooth setae (S5, para-anal setae) except slightly serrated Jv5 (Figures 9B and 10B). Peritreme short 63–76, extending to level of anterior margin of coxae IV (Figures 9B and 10B).

Gnathosoma — (Figures 9 and 10) — Shape of epistome similar as in larva (Figures 9D and 10C). Subcapitulum with six transverse rows of denticles, with 11–21 tiny denticles in each row, five rows distinctly serrated, bottom row nearly smooth (Figure 9E). Four pairs of hypostomal setae: h3 77–90 > h1 49–60 > h2 34–45 > pc 30–39 (Figure 9C). Corniculi robust and bifurcated as for adults, horn-like outer part 39–48, hook-like inner part 15–18 (Figure 9C). Setal counts of palp digits 200–235: trochanter 1, femur 4 (al slightly thickened serrated), genu 5 (al barbed), tibia 10, tiny spine on anterior margin of genu, genu all setae barbed; palp apotele two-tined (Figures 9C and F). Cheliceral digits 114–122 similar as larva (Figure 9G).

Legs — (Figure 11) — Leg II–IV with well-developed claws and pulvilli, all legs setae slightly serrated. Legs length: Leg I 625–674 > IV 505–603 > II 425–504 > III 396–492. Chaetotaxy of Legs: Leg I (Figure 11A): coxa 2, trochanter 4, femur (10) 2-2/1, 2/1-2, genu (8) 1-2/1, 2/1-1, tibia (8) 1-2/1, 2/1-1. Leg II (Figure 11B): coxa 2, trochanter 4, femur (8) 1-2/1, 2/1-1 (pd1 extended), genu (6) 1-2/0, 2/0-1 (pd1 extended), tibia (7) 1-1/1, 2/1-1. Leg III (Figure 11C): coxa 2, trochanter 4, femur (5) 1-2/1, 1/0-0 (pd1 extended), genu (6) 1-2/0, 2/0-1 (ad1 extended), tibia (7) 1-1/1, 2/1-1. Leg IV (Figure 11D): coxa 1, trochanter 4, femur (4) 1-1/0, 1/0-1, genu (5) 1-2/0, 2/0-0 (ad1 extended), tibia (7) 1-1/1, 2/1-1; extended macro setae ad4, pd4 on tarsus II–IV, ventral intercalary sclerite present on tarsus II–IV, only tarsus IV with ventral setae av4 and pv4 on ventral intercalary sclerite (Figures 11B, C and D).

Description (deutonymph)

(Figures 12–14)

Six specimens measured.

Dorsal idiosoma — (Figures 12 and 13) — Dorsal idiosoma 688–1093 long × 405–678 wide similar as for protonymph, covered with podonotal, mesonotal, and pygidial shields (Figures 12A and 13A). Podonotal shield 394–455 long × 332–429 wide bearing 12 pairs of smooth setae (j3–j6, z3–z6, and s3–s6); mesonotal shield 64–106 long × 267–340 wide bearing five pairs of smooth setae (J1, J2, and Z1–Z3); pygidial shield 120–177 long × 251–277 wide fused with ventrianal shield, dorsal pygidial region covered with distinct punctures with a pair of J5 on apical tubercle 170–208, smooth dorsal setae j1, j2, z1, z2, s1, and r1 on soft cuticle in podonotal region (Figure 13A).

Ventral idiosoma — (Figures 12 and 13) — Tritosternum with a pair of pilose laciniae 110–154, columnar base 63–84 long (Figures 12B and 13B). Sternal shield 257–300 long × 102–146 wide bearing st1 42–55, st2 36–45 and a pair of poroids iv1; sternal setae st3 37–49 and st4 20–38 on weakly sclerotized plates in soft cuticle; iv2, and iv3 on soft cuticle without plates (Figures 12B and 13B). Opisthogastric soft cuticle with nine pairs of smooth setae including st5, Jv1–Jv3; well-sclerotized metapodal shields covered with lineate, large pore-like punctate reticulation with Zv1 23–36. Peritreme short 194–239, extended to level of posterior margin of coxa II (Figures 12B and 13B). Ventrianal region of pygidial-ventrianal shield bearing four pairs of smooth setae (Figure 13B).

Gnathosoma — (Figures 12 and 13) — Shape of epistome similar to larva (Figures 12D and 13C). Subcapitulum with five transverse rows of denticles, each row with 26–30 tiny denticles (Figure 12E). Four pairs of hypostomal setae smooth: h3 99–115 > h1 60–82 > h2 46–80 > pc 38–52 (Figure 12C). Corniculi robust and bifurcated as adults, horn-like outer part 53–68, hook-like inner part 22–28 (Figure 12C). Setal counts of palp digits 264–318: trochanter 2 (al serrated), femur 5 (al slightly thickened serrated), genu 7 (al1 barbed, al2 spatulated), tibia 15, most of palpal setae slight serrated, small projection on anterior margin of genu (Figure 12F); palp apotele two-tined (Figures 12C and F). Cheliceral digits 143–162 well-developed, similar to female, excrescences hair-like (Figure 12G).

Legs — (Figure 14) — Leg II–IV with well-developed claws and pulvilli, most leg setae slightly serrated. Legs length: Leg I 831–946 > IV 692–979 > II 587–717 > III 567–716. Chaetotaxy of Legs: Leg I (Figure 14A): coxa 2, trochanter 6, femur (12) 2-2/1, 2/3-2, genu (12) 2-3/1, 3/1-2, tibia (14) 2-3/2, 3/2-2. Leg II (Figure 14B): coxa 2, trochanter 5, femur (10) 2-2/1, 2/2-1 (pd1 extended), genu (12) 2-3/1, 3/1-2 (pd1 extended), tibia (10) 2-2/1, 2/1-2. Leg III (Figure 14C): coxa 2, trochanter 5, femur (7) 1-2/1, 2/1-0, genu (10) 2-3/1, 2/1-1 (ad1 extended), tibia (9) 2-2/1, 2/1-1, Leg IV (Figure 14D): coxa 1, trochanter 5, femur (8) 1-2/1, 2/1-1, genu (10) 2-3/1, 3/0-1 (ad1 extended), tibia (10) 2-2/2, 2/1-1; long macro setae ad4 and pd4 on tarsus II–IV, ventral intercalary sclerite present on tarsus II–IV, only tarsus IV with ventral setae av4 and pv4 on ventral intercalary sclerite (Figures 14B, C and D).

Differential diagnosis

The three Asian species (S. bifurcata, S. cordata, and S. leptaulax n. sp.) form a distinct group, being the only species that have latigynal shields lacking pores and with fewer than three pairs of setae, and the large pore-like structure on the anterior margin of the ventral shield. Compared with the two species from China, S. leptaulax n. sp. is close to S. bifurcata in having distinctly bifurcate corniculi and hypertrichy of the posterior sternal margin, which bears four pairs of small setae (st4–st7). However, S. leptaulax n. sp. is different as having dorsal seta z4, z6, and J2 (absent in S. bifurcata) (Figures 15C and D), males with two pairs of short setae associated with the genital area, a pair of setae on the genital plate and the other pair of setae on the margin of the plate (S. bifurcata possesses a pair of seta only on the margin, not on the genital plate).

Etymology

The species name is derived from the genus name of the host passalid beetle Leptaulax koreanus Nomura, Kon, Johki & Lee, 1993 to emphasize the interaction between the mite and its host.

Key to species of Stenosternum Kramer, 1898 (modified from Zhang et al., 2017)

1. Latigynal shields without pores and with fewer than three pairs of setae; anterior margin of ventral shield with large pore-like structure
...... 2 (Asian)

— Latigynal shields with numerous pores and at least five pairs of setae; anterior margin of ventral shield without large pore-like structure
...... 4 (African, South American)

2. Latigynal shields with three pairs of setae; sternal shield with six pairs of setae; al2 setae on palp genu spatulate; corniculus significantly bifurcated; ventral shield with eight pairs of elongated setae; ventrianal shield bearing three pairs of setae
...... 3

— Latigynal shields with two pairs of setae; sternal shield with three pairs of setae; al2 setae on palp genu scimitar-shaped; corniculus slightly bifurcated; ventral shield with 24–25 short setae; ventrianal shield bearing five pairs of setae
...... S. cordata

3. Dorsal shield bearing 29 pairs of setae plus single j2 seta; in male, a pair of setae on the margin of genital opening
...... S. bifurcata

— Dorsal shield bearing 32 pairs of setae plus single j2 seta; in male, two pairs of setae on the margin of genital opening
...... S. leptaulax n. sp.

4. Latigynal shields appearing separate, with sclerotized medial margins
...... S. armiger

— Latigynal shields fused, without clear medial margins
...... 5

5. Latigynal shield truncate posteriorly, bearing nine pairs of setae; pore area extends over entire posterior portion of latigynal shield
...... S. bruneti

— Latigynal shield concave posteriorly; pore areas variable
...... 6

6. Latigynal shield bearing five pairs of setae; pores areas in two circular areas near posteriormargin of latigynal shield
...... S. tragardhi

— Latigynal shield bearing more than five pairs of setae; pose areas not in circular areas
...... 7

7. Ventral shield bearing seven pairs of setae; dorsal shield bearing 36 pairs of setae, with one pair adjacent, much longer than the other dorsal setae
...... S. bicrinus

— Ventral shield bearing 17–40 pairs of setae; dorsal shield bearing 23 pairs of setae, with one pair of adjacent, posteromarginal dorsal setae only slightly longer than the other dorsal setae
...... S. truittae

Discussion

This new species is the first trigynaspid mite from Korea and also the third species of Hoplomegistidae recorded in Asia. Zhang et al. (2017) made a separation between Asian species and other African and American species by the presence of the large pore-like structures on the anterior region of the ventral shield. The descriptions of most non-Asian species are rudimentary, being shown only for H. bicrinus and H. truittae (Hyatt 1964; Butler and Hunter 1966). However, examination of additional unidentified specimens from South America revealed that similar structures were present in all the female specimens, although may not have been depicted in descriptions as the external pore may be less well sclerotised (personal observation, OD Seeman). During the comparison between our new species and the two Chinese species, we discovered some aspects that require correction to help identify the Asian species correctly.

According to Zhang et al. (2017): I) in female S. bifurcata, the anterior tip of the endopodal plates are separated from the latigynal shield, II) in male S. bifurcata, no setae are on the margin of the genital opening, III) in male S. bifurcata, iv3 (stp3) is situated on posterior margin of sternogynal shield, and IV) both S. bifurcata and S. cordata were described with nine pairs of setae on femur II.

However, based on the re-examination of paratype specimens (Figure 15) we found that i) in female S. bifurcata, the anterior tip of endopodal plates fused with latigynal shield (Figures 15B and E, blue circle), ii) in male S. bifurcata, a pair of setae is situated on the margin of the genital opening (Figure 15F, red circle), iii) in male S. bifurcata, iv3 (stp3) is located right behind sternal setae st6 (Figure 15F), and iv) both S. bifurcata and S. cordata have 10 setae on femur II (personal communication, Jing-Lan Sun, without microphotographs), the same as in the Megisthanidae (Seeman 2017, 2019). Although some revisions were needed, the two Chinese species still possess a suite of features distinctive of the Asian Hoplomegistidae: bifurcate corniculi, less than five pairs of setae on the latigynal shield, and pore-like structures on the anterior margin of the ventral shield.

Acknowledgements

We are appreciative to Dr. Cheol-Min Kim for kindly helping identification of this species and also discussing the research idea and methods at the beginning of the study; and also, would like to thank Dr. Owen Seeman (Queensland Museum, Australia) for his highly perceptive comments on the manuscript; and Jing-Lan Sun (Guizhou University, China) for taking microphotographs of the type specimens; lastly, Jiwon Son, Seongwon Yoon, and Wonwoong Kim for helping with our fieldwork. This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. RS-2024-00405751), supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education (NRF2020R1I1A2069484, NRF-2018R1A6A1A03024862, and NRF-RS-2025-00561722), and also supported by a grant from the National Institute of Biological Resources (NIBR), funded by the Ministry of Environment (MOE) of the Republic of Korea (NIBR202002204). Omid Joharchi's study was also supported by the Russian Science Foundation, grant No. 24-46-00024 (https://rscf.ru/en/project/24-46-00024/ ).

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instructions